Add CI for all targets in doc/SupportPolicy.md

#584 (closed) highlights how we haven't actually tried to build arti on some important targets, like iOS. Our CI should make sure to cross-compile arti for each of these supported targets, and eventually run the tests as well.

(See #584 (comment 2835542) for a list of iOS targets to check against.)

Related to #303 (closed).

(This feels like something there should already be a ticket for, but I couldn't find one. Feel free to close this as a dupe.)